The present invention relates to a shredder provided with a shredding mechanism driven for shredding wastepaper by a motor and, more particularly, to a driving and controlling system for driving and controlling a shredding mechanism included in a shredder and capable of simultaneously shredding a small to a large number of paper sheets.
A generally known shredder comprises an induction motor, i.e., ac motor, driven by power supplied from a commercial power supply system of single-phase 100 V ac and 50 or 60 Hz, a reduction gear connected to the output of the motor and capable of reducing an input speed at a predetermined reduction ratio to a lower output speed and of increasing an input torque to a higher output torque, and a shredding mechanism, such as a rotary cutting mechanism, connected to the output side of the reduction gear.
FIG. 6 shows the speed-torque characteristic of the induction motor. The induction motor operates stably in a speed range between a point P6 corresponding to synchronous speed and a point P7 corresponding to stall torque. Therefore, the motor is operated in operating conditions represented by a line between the points P6 and P7. When load on the motor increases, the slip of the motor increases, current supplied to the motor increases and, consequently, a high torque is produced for shredding.
When the number of superposed paper sheets to be shredded simultaneously is increased and load on the induction motor, i.e., ac motor, employed in the known shredder is increased, the operating condition of the motor changes from the side of the point P6 corresponding to the synchronous speed toward the side of the point P7 corresponding to the stall torque and the current increases exponentially. Since power is supplied from a commercial power supply system to the motor, the voltage applied to the motor is unchangeable. Therefore, input power to the motor increases sharply in proportion to the current. To provide for such a condition, a lead-in cable connecting the shredder to the commercial power supply system must have a capacity large enough to withstand the high current.
If the shredder is overloaded and nothing is done to rectify the undesirable condition, the power supplied to the shredder will increase beyond the rated input power of the shredder or the operating speed of the motor decreases below the speed corresponding to the point P7 corresponding to the stall torque and stalls and the shredder is unable to exercise its function. Moreover, a current exceeding a current specified by the electrical appliance regulation law will flow through the service outlet and a circuit breaker will open the corresponding circuit to protect electrical appliances other than the shredder from overcurrents.
Generally, to avoid such a condition, the shredder is provided with means for stopping the motor before the torque of the motor reaches the stall torque at the point P7 and reversing paper sheets taken into the shredding mechanism to return the paper sheets to the feed side. If the shredder is thus reversed, shreds and scraps of the paper sheets subjected to shredding scatter in the shredding mechanism and around a feed unit, necessitating cleaning work.
Since a condition where the shredder is unable to function normally occurs if an excessively large number of paper sheets are fed simultaneously in a pile into the shredder, a reduced number of paper sheets are fed in a pile. However, if the shredder is used generally by unspecified people who do not necessarily have sufficient knowledge of the functions of the shredder, the interruption of shredding operation will occur very often, making the operation of the shredder very complicated. Those problems are attributable to a fact that it is difficult to control the operating speed and the torque of the induction motor optionally.
Furthermore, the conventional shredder employing an induction motor, i.e., an ac motor, has the following problems.
FIG. 9 shows the operating characteristic of a shredder provided with an induction motor as a driving means. FIG. 9 shows the variation of the operating speed of the motor with load loaded on the motor by wastepaper. In a nonloaded state P1 where any wastepaper is not fed to the shredder, the motor operates at an operating speed N1. In a fully loaded state P2 the motor operates at an operating speed N2. The slip of the induction motor increases when wastepaper is fed to the shredder and an actual load increases, and the operating speed decreases from N1 to N2. As obvious from FIG. 9, the induction motor has three states, i.e., a waiting state after the start of the motor in which wastepaper is not fed yet, a shredding state where the motor is operating in conditions between the states P1 and P2 on the characteristic curve and a stopped state where power is not supplied to the motor. The motor is operating at high operating speeds in most of the time whether or not wastepaper is fed to the shredder.
The induction motor is designed so as to operate at a high efficiency in a high-load region to operate under a maximum shredding load by using a current not exceeding a limit current of the commercial power supply system. Therefore, the efficiency of the induction motor is low when the induction motor operates in a low-load region. FIG. 10 shows the load-torque characteristic and the load-current characteristic of an induction of such a design. Suppose that the torque of the motor is T3 and a current I3 is supplied to the motor when the motor is in a state corresponding to a low-load point P3, and the torque of the motor is T4 and a current I4 is supplied to the motor when the motor is in a state corresponding to a high-load point P4. Then,
(T3/I3) less than (T4/I4)xe2x80x83xe2x80x83(1)
When the motor is in the state corresponding to the high-load point P4, a voltage drop across the winding due to the resistance of the winding and the current I4 necessary for producing the torque T4 is large. The design of the winding of the motor is determined in anticipation of such a large voltage drop. However, although the intensity of the current I3 necessary for producing the torque T3 is low and the voltage drop across the winding is small when the motor is in the state corresponding to the low-load point P3, unnecessary power is supplied to the motor because the voltage applied to the motor is fixed and hence the efficiency is low. In FIG. 10, a curve A indicates motor current, a curve B indicates torque, and a curve C indicates excitation current.
FIG. 11 is a graph showing the variation of starting current required by an induction motor at start with time. A high starting current flows in a period between motor start time t8 and time t9 when the operating speed stabilizes.
The conventional shredder employing the induction motor, i.e., ac motor, has the nonloaded state P1 where any wastepaper is not fed to the shredder, the loaded state P1-P2 where the shredder is loaded with wastepaper and the operating speed of the motor decreases from N1 to N2, and the fully loaded state P2 where the shredder is operating under a full load condition as shown in FIG. 9. However, the motor is operating at high operating speeds in most of the time whether or not wastepaper is fed to the shredder. Therefore, the motor operates at substantially fixed high operating speeds regardless of load, so that the motor and the shredding mechanism generate noise and vibrations, which deteriorates environmental conditions significantly.
Since importance is attached to the high-load state when designing the induction motor for the shredder, the induction motor operates at the highest efficiency under a high load. Therefore, in the nonloaded state or the low-load state where the shredder is not operating for shredding, the motor operates at a low efficiency as shown in FIG. 10 and consumes much power wastefully. As is obvious from the relation between the motor current indicated by the curve A and the excitation current indicated by the curve C in FIG. 10, the ratio of the excitation current to the motor current in the state corresponding to the low-load point P3 is greater than that in the state corresponding to the high-load point P4.
Even if the motor is kept stopped while the shredder not in the shredding operation to suppress power consumption, the motor needs to be started every time wastepaper is fed to the shredder and the high starting current flows frequency and hence power consumption cannot be effectively reduced. If the high starting current as shown in FIG. 11 flows frequently, the motor is overheated to reduce the efficiency and the overheating of the motor creates a danger to the operator. Thus, any effective means have not been available.
To solve the foregoing problems, the present invention provides a driving and controlling system for a shredder for shredding paper sheets comprises a motor for driving a shredding mechanism for shredding paper sheets, and a control means interposed between the motor and a commercial power supply system to control power supplied to the motor, wherein the motor has a characteristic to reduce motor torque substantially linearly with the increase of operating speed for voltage applied thereto as a parameter, and the control means controls the motor so that the operating speed of the motor decreases as the necessary torque of the shredding mechanism required for shredding paper sheets increases, and the power supplied from the commercial power supply system may not increase beyond a predetermined level.
According to the present invention, the control means reduces the operating speed of the motor when supply power supplied to the control means from the commercial power supply system increases with the increase of the necessary torque of the shredding mechanism and approaches an allowable limit level to reduce input power supplied from the control means to the motor, maintaining a desired motor torque. Consequently, the supply power supplied from the commercial power supply system to the control means decreases. When the input power approaches the allowable limit level again, the operating speed is reduced further to maintain the same operation. When reducing the operating speed, the control means reduces principally the voltage component of the input power supplied from the control means to the motor. Therefore, the motor current can be increased, and an allowable motor torque at input power below the allowable limit level for the commercial power supply system increases with the decrease of the operating speed.
According to the present invention, the operating speed is reduced so that the input current is below the allowable limit level to make the motor produce a maximum torque at all times, and the number of paper sheets that can be simultaneously shredded can be increased, keeping the input current from the commercial power supply system below the allowable limit level, so that the frequency of interruption of the shredding operation due to overloading can be greatly reduced. According to the present invention, a combination of a maximum operating speed and a maximum torque can be selectively determined and controlled while the input current is kept below the allowable limit level. Therefore, a shredding time necessary for shredding the same number of paper sheets can be reduced. Since the operating speed can be increased to a maximum according to a necessary torque, keeping the supply current from the commercial power supply system below the allowable limit level, shredding speed can be increased beyond a conventional synchronous speed. Since a shredding speed can be previously determined regardless of the frequency of the commercial power supply system, the shredding mechanism need not be replaced with another one when the frequency of available power changes and the shredding characteristic remains unchanged regardless of frequency.
According to the present invention, the driving and controlling system is provided with a sheet detecting means for detecting paper sheets fed to the shredding mechanism, and the control means changes the operating mode of the motor on the basis of the result of a detecting operation of the sheet detecting means. The present invention solves the technical problems in the conventional shredder attributable to the employment of the induction motor, the shredder is set in a low-speed operating state or a stopping state when any paper sheets are not fed-to the shredding mechanism, noise and vibrations generated by the operating motor and the operating shredding mechanism can be reduced, power consumption is suppressed, energy can be saved and noise can be reduced.